1. Field of the Invention
The present invention relates generally to computer networks, and more specifically, to a method and apparatus for preventing the formation of loops.
2. Background Information
A computer network typically comprises a plurality of interconnected entities. An entity may consist of any device, such as a computer or end station, that “sources” (i.e., transmits) or “sinks” (i.e., receives) data frames. A common type of computer network is a local area network (“LAN”) which typically refers to a privately owned network within a single building or campus. LANs typically employ a data communication protocol (LAN standard), such as Ethernet, FDDI or token ring, that defines the functions performed by the data link and physical layers of a communications architecture (i.e., a protocol stack). In many instances, several LANs may be interconnected by point-to-point is links, microwave transceivers, satellite hook-ups, etc. to form a wide area network (“WAN”) or intranet that may span an entire country or continent.
One or more intermediate network devices are often used to couple LANs together and allow the corresponding entities to exchange information. For example, a bridge may be used to provide a “bridging” function between two or more LANs. Alternatively, a switch may be utilized to provide a “switching” function for transferring information between a plurality of LANs or end stations. Typically, the bridge or switch is a computer and includes a plurality of ports that couple the device to the LANs or end stations. The switching function includes receiving data from a sending entity at a source port and transferring that data to at least one destination port for forwarding to the receiving entity.
Switches and bridges typically learn which destination port to use in order to reach a particular entity by noting on which source port the last message originating from that entity was received. This information is then stored by the bridge in a block of memory referred to as a filtering database. Thereafter, when a message addressed to a given entity is received on a source port, the bridge looks up the entity in its filtering database and identifies the appropriate destination port to reach that entity. If no destination port is identified in the filtering database, the bridge floods the message out all ports, except the port on which the message was received. Messages addressed to broadcast or multicast addresses are also flooded.
Additionally, most computer networks are either partially or fully meshed. That is, they include redundant communications paths so that a failure of any given link or device does not isolate any portion of the network. The existence of redundant links, however, may cause the formation of circuitous paths or “loops” within the network. Loops are highly undesirable because data frames may traverse the loops indefinitely. Furthermore, because switches and bridges replicate (i.e., flood) frames whose destination port is unknown or which are directed to broadcast or multicast addresses, the existence of loops may cause a proliferation of data frames so large that the network becomes overwhelmed.
Spanning Tree Protocol
To avoid the formation of loops, most bridges and switches execute a spanning tree protocol or algorithm which allows them to calculate an active network topology that is loop-free (i.e., a tree) and yet connects every pair of LANs within the network (i.e., the tree is spanning). The Institute of Electrical and Electronics Engineers (IEEE) has promulgated a standard (the 802.1D standard) that defines a spanning tree protocol to be executed by 802.1D compatible devices. In general, by executing the 802.1D spanning tree protocol, bridges elect a single bridge within the bridged network to be the “root” bridge. The 802.1D standard takes advantage of the fact that each bridge has a unique numerical identifier (bridge ID) by specifying that the root is the bridge with the lowest bridge ID. In addition, for each LAN coupled to more than one bridge, only one (the “designated bridge”) is elected to forward frames to and from the respective LAN. The designated bridge is typically the one closest to the root. Each bridge also selects one port (its “root port”) which gives the lowest cost path to the root. The root ports and designated bridge ports are selected for inclusion in the active topology and are placed in a forwarding state so that data frames may be forwarded to and from these ports and thus onto the corresponding paths or links of the network. Ports not included within the active topology are placed in a blocking state. When a port is in the blocking state, data frames will not be forwarded to or received from the port. A network administrator may also exclude a port from the spanning tree by placing it in a disabled state.
To obtain the information necessary to run the spanning tree protocol, bridges exchange special messages called configuration bridge protocol data unit (BPDU) messages. More specifically, upon start-up, each bridge initially assumes that it is the root and transmits BPDU messages accordingly. Upon receipt of a BPDU message from a neighboring device, its contents are examined and compared with similar information (e.g., assumed root and lowest root path cost) stored by the receiving bridge. If the information from the received BPDU is “better” than the stored information, the bridge adopts the better information and uses it in the BPDUs that it sends (adding the cost associated with the receiving port to the root path cost) from its ports, other than the port on which the “better” information was received. Although BPDU messages are not forwarded by bridges, the identifier of the root is eventually propagated to and adopted by all bridges as described above, allowing them to select their root port and any designated port(s).
In order to adapt the active topology to changes and failures, the root periodically (e.g., every hello time) transmits BPDU messages. The default hello time is two seconds. In response to receiving BPDUs on their root ports, bridges transmit their own BPDUs from their designated ports, if any. Thus, every two seconds BPDUs are propagated throughout the bridged network, confirming the active topology. If a bridge stops receiving BPDU messages on a given port (indicating a possible link or device failure), it will continue to increment a respective message age value until it reaches a maximum age (max age) threshold. The bridge will then age out, i.e., discard, the stored BPDU information and proceed to re-calculate the root, root path cost and root port by transmitting BPDU messages utilizing the next best information it has. The maximum age value used within the bridged network is typically set by the root, which enters the appropriate value in its BPDU messages. Normally, each bridge replaces its stored BPDU information every hello time, thereby preventing it from being discarded and maintaining the current active topology.
When BPDU information is updated and/or timed-out and the active topology is re-calculated, ports may transition from the blocking state to the forwarding state and vice versa. That is, as a result of new BPDU information, a previously blocked port may learn that it should be in the forwarding state (e.g., it is now the root port or a designated port). Rather than transition directly from the blocking state to the forwarding state, the 802.1D standard calls for ports to transition through two intermediate states: a listening state and a learning state. In the listening state, a port waits for information indicating that it should return to the blocking state. If, by the end of a preset time, no such information is received, the port transitions to the learning state. In the learning state, a port still blocks the receiving and forwarding of frames, but received frames are examined and the corresponding location information is stored in the bridge's filtering database. At the end of a second preset time, the port transitions from the learning state to the forwarding state, thereby allowing frames to be forwarded to and from the port. The time spent in each of the listening and the learning states is referred to as the forwarding delay.
Although the spanning tree protocol provided in the 802.1D standard is able to maintain a loop-free topology despite network changes and failures, re-calculation of the active topology can be a time consuming and processor intensive task. For example, re-calculation of the spanning tree following an intermediate device crash or failure can take approximately thirty seconds. During this time, message delivery is often delayed as ports transition between states. Such delays can have serious consequences on time-sensitive traffic flows, such as voice or video traffic streams.
Rapid Spanning Tree Protocol
Recently, the IEEE promulgated a new standard (the 802.1w standard) that defines a rapid spanning tree protocol (RSTP) to be executed by otherwise 802.1D compatible devices. The RSTP similarly selects one bridge of a bridged network to be the root bridge and defines an active topology that provides complete connectivity among the LANs while severing any loops. Each individual port of each bridge is assigned a port role according to whether the port is to be part of the active topology. The port roles defined by the 802.1w standard include Root, Designated, Alternate and Backup. The bridge port offering the best, e.g., lowest cost, path to the root is assigned the Root Port Role. Each bridge port offering an alternative, e.g., higher cost, path to the root is assigned the Alternate Port Role. Each bridge port providing the lowest cost path from a given LAN is assigned the Designated Port Role, while all other ports coupled to the given LAN in loop-back fashion are assigned the Backup Port Role.
Those ports that have been assigned the Root Port and Designated Port Roles are placed in the forwarding state, while ports assigned the Alternate and Backup Roles are placed in a discarding or blocking state. A port assigned the Root Port Role can be rapidly transitioned to the forwarding state provided that all of the ports assigned the Alternate Port Role are placed in the discarding or blocking state. Similarly, if a failure occurs on the port currently assigned the Root Port Role, a port assigned the Alternate Port Role can be reassigned to the Root Port Role and rapidly transitioned to the forwarding state, provided that the previous root port has been transitioned to the discarding or blocking state. A port assigned the Designated Port Role or a Backup Port Role that is to be reassigned to the Designated Port Role can be rapidly transitioned to the forwarding state, provided that the roles of the ports of the downstream bridge are consistent with this port being transitioned to forwarding. The RSTP provides an explicit handshake to be used by neighboring bridges to confirm that a new designated port can rapidly transition to the forwarding state.
Like the STP described in the 802.1D specification standard, bridges running the RSTP also exchange BPDU messages in order to determine which roles to assign to the bridge's ports. The BPDU messages are also utilized in the handshake employed to rapidly transition designated ports to the forwarding state. RSTP also uses timers, including a received information while (rcvdInfoWhile) timer, which is similar to STP's max age timer. The rcvdInfoWhile timer is a count down (to zero) timer, while the max age timer is a count up timer.
Loops Undetectable by Spanning Tree Protocols
In some cases, a single, duplex link coupling two neighboring bridges (which are also indirectly coupled through other bridges or devices) may physically comprise two simplex, i.e., unidirectional, transmission lines, such as two fiber optic lines, operating in opposite directions. Certain failures associated with such lines can result in the formation of loops that are undetectable by the STP. For example, suppose two bridges, designated A and B, are connected by a single trunk link formed from two unidirectional transmission lines, and that the respective port at Bridge B is assigned the designated port role, while the peer port at Bridge A is assigned the alternate port role. In this case, the port at Bridge B is placed in the forwarding state and the port at bridge A is placed in the discarding state. As long as the port at Bridge A continues to receive “superior” BPDU messages from Bridge B, it will remain in the blocking state. Suppose, however, that the trunk link becomes unidirectional. That is, bridge B continues to send BPDU messages to Bridge A, but these BPDU messages are never received, and yet the trunk line is not considered to be “down”. Accordingly, the BPDU information stored for the port at Bridge A eventually ages out and the STP running at Bridge A transitions the port to the forwarding state. Because Bridge B is unaware of the link failure, the port at Bridge B remains in the forwarding state. With the ports at both Bridge A and Bridge B in the forwarding state a loop is created. As described above, the creation of such a loop causes network messages to be replicated, wasting substantial network bandwidth and potentially causing a network outage.
A loop may also be created as a result of an error or failure in the operation of the STP at Bridge B, such as a software error. Specifically, the STP running at Bridge B may determine that the port of Bridge B that is coupled to Bridge A should be assigned the Designated Port Role and be transitioned to the forwarding state. Yet, the STP running at Bridge B may fail for some reason to have BPDU messages sent from the port. In this case, the STP running at Bridge A concludes that its port should now be assigned the designated port role and that it should be transitioned to the forwarding state. With the ports at both Bridge A and Bridge B in the forwarding state, a loop is created. Certain hardware failures can similarly result in the creation of loops. For example, the STP running at Bridge B may generate BPDU messages for transmission from the port coupled to Bride A, but those BPDU messages may never get sent due to a hardware problem at Bridge B.
In summary, unidirectional failures resulting in the formation of loops may occur as a result of malfunctioning or faulty network interface cards (NICs) and/or transceivers, a switch's central processing unit (CPU) being too busy with other processes to send BPDU messages for a relatively long time, a software bug in the STP running at the switch, or congestion algorithms that end up dropping BPDU messages. In addition, if a link up/down detection and/or autonegotiation protocol is disabled, e.g., by network administrator action, unidirectional failures may go undetected, resulting in loops. Accordingly, a need exists to prevent the formation of loops that are undetectable by the STP.